The invention relates to a method for obtaining a crystallized material in presence of a magnetic field. It generally applies to any material presenting a magnetic anisotropy in the crystal state although it is more specifically disclosed hereinafter in connection with the manufacturing of high temperature superconducting materials such as materials of the RBaCuO family where R is a rare earth element such as yttrium.
Many authors have also indicated the possibility and the advantages of applying a magnetic field during sintering for improving the orientation or the size of crystalline grains to be obtained.
A part of the prior art documents relates to crystallization improvement when the material to be obtained is placed in an non-homogeneous medium. An example of such a prior art document is article of A. E. Mikelson and al. published in the Journal of Crystal Growth, 52, 1981, pages 524-529. In this article, it is taught that, if a magnetic field of about one tesla is applied to a cadmium-zinc alloy in the molten state, one obtains cadmium-zinc dendrites in an alloy having a different composition. These dendrites are oriented along the field lines.
In other articles, which more particularly deal with obtaining high temperature superconductors, it is suggested to make a superconductive ceramic by sintering and pressing, then annealing in presence of a field for improving the crystalline structure of the ceramic. So, in the European Patent Application 0 284 534, the first example relates to YbaCuO manufacturing and three successive annealing are disclosed, the first between 500.degree. and 1200.degree. C., for example 700.degree. C., the second between 500.degree. and 1200.degree. C., for example 900.degree. C., and the third between 600.degree. and 1200.degree. C., for example 800.degree. C. So, in all the specific examples disclosed, although the temperature range indicated by the applicant reaches the melting temperature of YBaCuO (close to 1150.degree. to 1200.degree. C.), it is taught to stay below this temperature and no difference is made between what occurs below and above the melting temperature.
The present invention teaches a method comprising applying a magnetic field to a material during the crystallization or recrystallization, wherein this magnetic field is applied while the material is in a liquid phase and comprises crystallites liable to constitute crystallization seeds when cooling.
It will be shown that the implementation of such a method permits to obtain a polycrystalline composition wherein the grains are much better oriented than with the conventional methods and in particular when the field application is not made while the substance is in a liquid phase.
Before explaining in greater detail the invention, some general law of magnetism will be recalled.
Magnetic materials have a magnetic susceptibility .chi. which is generally anisotropic. For example, there are materials that have an axis of easy magnetization, hereinafter called axis c, the two other axes being axes a and b. Thus, if .chi. is the magnetic susceptibility, the difference in magnetic suspectibility between the axis of easy magnetization (c) and the hard magnetization directions (a and b), is: EQU .DELTA..sub..chi. =.sub..chi.c -.sub..chi.ab
If a magnetic field B is applied, particles tend to be oriented according to their axis of easy magnetization and an energy gain .DELTA.E is produced with respect to the case of a material with a random distribution of the magnetic axes: EQU .DELTA.E=V.B.sup.2..DELTA..sub..chi. /2.sub..mu.0
where V is the volume considered and .mu..sub.0 =4.pi..10.sup.-7 in international units (I.U.).
If it is desired to orientate a magnetic material in a field, this energy gain .DELTA.E must be substantially higher than the energy associated with the thermal agitation, namely, kT, where T is the absolute temperature and k the Boltzmann's constant.
The result of this comparison gives the definition of volumes or elementary domains liable to be satisfactorily oriented. For example, for a YBa.sub.2 Cu.sub.3 O.sub.7 grain of 1 .mu.m.sup.3, which constitutes a high temperature superconductor, .DELTA..sub..chi. will be about 10.sup.-5 I.U. which gives .DELTA.E/kT=10.sup.4 at T=1500.degree. K. and for B=5 teslas, that is, .DELTA.E.gtoreq.kT. But, .DELTA.E/kT is equal to 10 only if the grain size decreases to 10.sup.-3 .mu.m.sup.3.
The simple case of an uniaxial anisotropy will be considered here. However, it is known that some magnetic materials may have several equivalent axes of easy magnetization and even an easy magnetization plane. This magnetic anisotropy may be very high when the material is magnetically ordered, particularly when it is ferromagnetic. In the paramagnetic state, the magnetic anisotropy is low but often sufficient for alignment under a magnetic field.
It will be reminded that the magnetic susceptibility .chi. varies with the inverse of the square temperature (1/T.sup.2), that is .chi. decreases quickly when T increases.